Electrophotographic recording member and process of producing same



nited States ELECTROPHOTOGRAPHIC RECORDING MEMBER AND PROCESS OFPRODUCING SAME Thomas J. Kucera, Fort Lee, N.J., assignor to CharlesBrunmg Company,Inc., Mount Prospect, 11]., a corporation of Delaware NoDrawing. Filed Dec. 18, 1958, Ser. No. 781,206 Claims. (Cl. 96-1)suspended in this solution. A preferred photo-conductor is zinc oxide,such, for example, as the photo-conductive zinc oxide sold by the NewJersey Zinc Company as Florence Green Seal 8. Other photo-conductorssuch as the oxides of antimony, aluminum, bismuth, cadmium, mercury,molybdenum, and lead; the iodides, selenides, sulfides or tellurides ofthese metals including zinc; selenium; arsenic trisulfide; lead chromateand cadmium arsenide have been suggested. The resinous vehicle forms anelectrically-insulating binder for the photo-conductor.

Equipment required for the application of such organic solvent systemsare usually costly, requiring solvent recovery systems and specialequipment to maintain the atmosphere in the neighborhood of the coatingappliances reasonably safe (i.e., no undue fire hazard) and nontoxic tothe workmen. While the organic solvent coating procedures result inelectrophotographic recording members having high dark resistivity,i.e., low dark decay, frequently they produce electrophotographicrecording members in which the adhesion between the resinous coatinglayer and the substrate is relatively poor.

Attempts heretofore made to apply the resinous vehicle or binderdispersed in an aqueous medium resulted in a coating of drasticallydiminished electrical insulating properties with consequent high darkdecay. Such electrophotographic recording members also exhibit markedlyincreased sensitivity to moisture.

It is among the objects of the present invention to provide a process ofproducing electrophotographic recording members, which process does notinvolve the use of an organic solvent for the resinous vehicle and hencerequires no special coating equipment, permitting the use ofconventional continuous web coating equipment without requiring anyadditional equipment for fire and health protection necessary whenemploying an organic solvent. 7

It is another object of this invention to provide such process employingan aqueous base for the resinous vehicle, which process results in aproduct having low dark decay, at least as. good as the productsproduced by the organic solvent method, and further having the resinousvehicle firmly bonded to the substrate, especially when a papersubstrate, as is preferred, is used.

It is still another object of the present invention to provide anelectrophotographic recording member in which the resinous vehicle isfirmly bonded. to the substrate and which has good dark decayproperties, permitting its storage for relatively long periods of timebefore use to produce excellent electrostatic copies, i.e., copieswhichwill readily receive an electrostatic charge ten on which a latent imageof an original can be produced by exposure to light, which image can berendered permanent by application of developer or toner followed byfixing of the image, all as is well known in the art of producingelectrostatic copies.

Other objects and advantages of this invention will be apparent from thefollowing description thereof.

In accordance with this invention the coating material applied to theelectrically-conducting or semi-conducting substrate consists of asuspension of the finely divided photo-conductor in an aqueous mediumcontaining an uncured melamine-formaldehyde resin having at least twomols of formaldehyde per mol of melamine, preferably at least 3 to 4mols of formaldehyde per mol of melamine and from 1 to 8 preferablyabout 3 parts by weight of photo-conductor per part of total resinsolids, and after application of this suspension to the substrate thecoating is heated to eifect curing of the melamineformal-' dehyde resin.When a flexible substrate such as paper.

is used, the suspension of the finely divided photo-conductor alsocontains a thermoplastic resin in amount of from 10% to preferably 35%to 55%, by weight of thermoplastic resin based on the weight of totalresin solids. Hence the present invention involves the application tothe substrate of a suspension containing on a dry basis (not includingthe water) from 2 to 50, preferably 7 to 35, parts by weight of uncuredmelamineformaldehyde resin, 0 to 40, preferably 6 to 30, parts by weightof thermoplastic resin and 50 to 88, preferably 50 to 85, parts byweight of photo-conductor.

The resultant electrophotographic recording member has the finelydivided photo-conductor particles embedded in an electrically-insulatinglayer consisting of the melamine-formaldehyde resin alone or uniformlyblended with the thermoplastic resin, when the latter resin is employed,which layer is firmly bonded with the substrate. The electrophotographicrecording member has excellent electrophotographic properties, includinglow or long-time dark decay, at least as good as products made byorganic solvent processes. a

As the substrate paper is preferred, including high wet strength papercoated or uncoated, having a thickness of from 3 to 6 mils. Otherelectrically-conductive or semi-conductive materials may be used, such,for example, as plastic films including cellophane, cloth, and metallicfoils, e.g., aluminum and copper foils.

The melamine-formaldehyde resin used may be dimethylol melamine (e.g.,Resloom HP of Monsanto- Chemical Co.); dimethyl trirnethylol melamine(Aerotex M-3 or Parez 613 of American Cyanamid Co.); trimethylolmelamine (Aerotex 605 or Parez 607 of American Cyanamid Co.);tetramethylol melamine (Resloom HP Special, Monsanto Chemical Co.);tetramethyl pentamethylol melamine (Resloom LC-48, Monsanto ChemicalCo.); tetramethyl hexamethylol melamine; pentamethyl pentamethylolmelamine; pentamethyl hexamethylol melamine; other alkylated melamineformaldehyde resins; or mixtures of such resins.

All of the above melamine formaldehyde resins contain at least 2 mols offormaldehyde per mol of melamine. It will be appreciated that the resinsmay be produced by reacting melamine with formaldehyde polymers oraqueous or alcoholic solutions of formaldehyde or formaldehyde polymersand that the reference to the number of mols of formaldehyde is to thenumber of mols present in the cured resin irrespective of the source ofthe formaldehyde. 2

The melamine-formaldehyde resin should be in a watersoluble condition,i.e., uncured or partially cured to a stage where it is stillappreciably water-soluble when applied to the substrate alone or inadmixture with the thermoplastic resin. It may be used with or Without ahydearid thermoplastic resin. Examples of such curing catalysts are zincchloride, magnesium chloride, ammonium chloride, acids such as acetic orhydrochloric acid, phenyl biguanide hydrochloride, anddiarnmonium-phosphate. Only a small amount of the catalyst need beemployed, enough tolcatalyze the curing of the resin.

Usually from 1% to 5% by weight based on the weight ofmelamine-formaldehyde resin will sufiice. The melamine-formaldehyderesin may be used alone when coating less flexible substrate such asmetal foils or plastic films. In the treatment'of flexible substratesuch as paper, the ,thermoplas'ticresin should be used along with themel-' amine formaldehyde resin; by so doing, cracking of the resincoating'u'pon flexureof the substarate is minimized, if not completelyprevented.

The thermoplastic resin should be compatible with themelamine-formaldehyde resin and blend therewith. By employing themixture, a resin coating resultswhich has good electrically-insulating"properties, is tough and ad heren't t0 the substrate, particularlypaper, and is not too brittle, i.e., will not crack or peel when thepaper or other flexible substrate is flexed or bent. Examples ofthermoplastic resins which can be used are:

(a) Vinylacetate'homopolymer (Gelva 8-55, Shawini- (b) Yinylacetateoctylacrylate copolymer (Resyn 2203,

-. National Starch) (c) ,[inyl acetate vinyl chloride copolymer (Resyn2507,

National starch) v (d Yinylidene chloride vinyl chloride copolymer(Latex 744 B,Dow Chemical) I (e) v.Vinylidene chloride acrylonitrilecopolymer (Saran F122- -Al5, Dow Chemical) (f) Acrylo nitrile-butadienecopolym'er (Hyca'r 1577,

. Goodrich) v t (g) Styrene-butadiene copolymer (Latex 512R, Dow

Chemical) (h) Interpolymers of 2-ethylhexyl acrylate, styrene,

acrylonitrile and methacrylic acid disclosed in United StatesPatent2,767,153 (Lytron 680, Monsanto Chemical Co.)

Preferred'resin mixtures are mixtures of Parez 613 and Gelva 8-55 inamount of from 35% to 55% by Weight or the Ge lva s-ss based on theweight of total resin formaldehyde resin with or without the aid of adispersing agent such as the alkali metal salts of alliyl naphthalenesjulfonicacids (Daxad No. llpf Dewey and Almy Co.,'

Qambridge, Massachusetts). The resultant dispersionis then mixed with anemulsion of the thermoplastic resin in water or a dispersion of thethermoplastic resin in water, in which dispersion the resin particlesare in extremely finely divided, e.g., colloidal, form so that they willblend readily with the melamine-formaldehyde resin. Alternatively themelamine-formaldehyde resin may be blended with the aqueous emulsion or.dispersion of the thermoplastic resin and the photo-conductor added tothe mixture while agitating.

The coating mixture thus prepared contians from 1 to 43 parts of resinsolids (melamine-formaldehyde, or mixture of melamine-formaldehyde andthermoplastic resin), from 10 to 85 parts of water, and from 10 to 75parts of photoeconductor, with the ratio of photo-conductor to resinsolids within the ranges of l to l to 8 to 1; The coating layer appliedto the substrate may have any de- 4. sired thickness; usually athickness within the range of .2 to 1 mil gives satisfactoryresults.

After application of the coating mixture to the substrate whichdesirably is in web form and which coating can be carried outefliciently in conventional coating equipment (spraying, immersion orcoating roll types) not requiring any special equipment to render thecoating operation safe from either a fire or health standpoint, thecoated substrate is subjected to a curing and drying treatment; Forexample, itmay be passed through a curing oven maintained at the curingtemperature. Curing maybe effected at any temperature above about 180 F.and below the temperature at which'charring or damage to the subtratemay take place. Preferred curing temperature is within the rangeof 180F. to 300 F., preferabiy about 240 F.250 F. During this curingtreatmentresidual moisture is removed from the insulating layer.

The time of cure will depend on the temperature and whether or not acatalyst is'employedi Satisfactory cure can be obtained in from 1 0 5minutes at 240 F.-250 F. without a curing catalyst in the coatingmixture. In general the curing time may vary from 1 to 15 minutes; at180 F. a longer curing time within this range is used and at about 250F. ashorter curing time of about 1 to 5 minutes is used.

The resultant electrophotographic recording element, particularly whenmade with a paper substrate, has the electrically-insulating resin layerfirmly bonded to the substrate and also has excellent electricalproperties. The dielectric properties are at least as good as productsmade employing organic solvent resin solutions, yet the procedure of thepresent invention involves none of the disadvantages inherent in anyprocedure involving the use of such organic solvents.

The following examples of coating procedures embodying the invention aregiven for illustrative purposes. It will be understood the invention isnot limited to these examples.

\ 7 Example I 600 parts of zinc oxide are dispersed in parts of Parez613 (80% solids, 20% water) employing 2 parts of a dispersing agent(Daxad No. 11). While agitating this dispersion 325 parts of polyvinylacetate resin emulsion (51% solids, 49% water) (Resyn 25-1234) areadded. After thorough mixing for about 45 minutes the dispersion iscoated on paperto a thickness of 0.005 inch. The coated paper is thenheated at 240 F.--250 F. for

5 minutes. e V An electrophotographic recording element is thus producedat least as good as the zinc oxide coated paper made from a solution ofsilicone resin in a toluene-xylene mixture.

. Example II This example difiers from Example I chiefly in that thethermoplastic resin employedis Resyn 2-507 and the proportion of the tworesin constituents is 25% melamineformaldehy'de resin to Resyn 2507. Theresults are the same as in Example I.-

Example III All of the photo-conductor. An eminently satisfactoryelectrophotographic recording element results.

Example V This example differs from Example I in that it involves theuse of lead sulfide as the photo-conductor and the use of a resinmixture of Parez 613 and Latex 512R containing equal parts of each resinin a ratio of 1 part resin solids, 1 part water, and 2 parts of the.photo-com;

ductor. An eminently satisfactory electrophotographic' recording elementresults.

Example VI This example differs from Example I in that it in volves theuse of zinc sulfide as the photo-conductor and the use of a resinmixture of Parez 613 and Latex 512R containing equal parts of each resinin a ratio of 1 part resin solids, 1 part water, and 2 parts of thephotoconductor. An eminently satisfactory electrophotographic recordingelement results.

Example VII Examples VIII, IX, X, XI

These examples difiier respectively from Example I in that instead ofthe 325 parts of polyvinyl acetate resin emulsion used in Example I,Example VIII involves the use of 300 parts of vinylidene chloride vinylchloride copolymer. Example IX involves the use of '350'parts ofvinylidene chloride acrylonitrile copolymer, Example X involves the useof acrylonitrile butadiene copolymer and Example H involves the use ofLytron 680, interpolymers of Z-ethylhexyl acrylate, styreneacrylonitrile and methacrylic acid. Otherwise the conditions are thesame and the results substantially the same.

It will be noted the present invention provides a process of producingelectrophotographic recording members free of the objections inherent inthe use of organic solvents, which process requires no special coatingequipment for its practice and no special safety or health precautions,and which process results in electrophotographic recording membershaving good electric-a1 properties including low dark decay and goodinsulating properties and also having the electrically-insulatingresinous layer firmly bonded to the substrate.

Since certain changes in carrying out the process and certainmodifications in the electrophotographic recording element embodyingthis invention may be made without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. The method of producing an electrophotographic recording member whichcomprises coating a substrate with a suspension of finely dividedelectrophotographic photoconductor from the group consisting of theoxides, iodides, selenides, sulfides and tellurides of zinc, antimony,aluminum, bismuth, cadmium, mercury, molybdenum and lead, selenium,arsenic trisulfide, lead chromate and cadmium arsenide in an aqueousmedium containing from 45% to 65% by weight of total resin solidspresent of a melamine-formaldehyde resin having at least 2 mols offormaldehyde per mol of melamine condensed therein and from 35% to 65 byWeight of total resin solids present of a thermoplastic resin from thegroup consisting of vinyl acetate homopolymers, vinyl acetateoctylacrylate copolymers, vinyl acetate vinyl chloride copolymers,vinylidene chloride vinyl chloride, copolymers, vinylidene chlorideacrylonitrile copolymers, acrylonitrile-butadiene copolymers,styrene-butadiene copolymers, interpolymers of 2-ethylhexyl acrylate,styrene, acrylonitrile and methacrylic acid, the ratio of saidphotoconductor to said resins in said suspension, on a dry basis, beingwithin the range of from 1 to 1 to 8 to 1 parts by weight, andthereafter heating the coated substrate to effect curing of themelamine-formeldehyde resin and produce a photo-conductive insulatinglayer having the photoconductor particles imbedded therein, said layerbeing continuous and firmly bonded to the substrate.

2. The method of producing an electrophotographic recording member whichcomprises coating a substrate with a suspension of finely divided zincoxide photoconductor in an aqueous medium containing from 45% to 65% byweight of total resin solids present of a melamine-formeldehyde resinhaving at least 2 mols of formaldehyde per mol of melamine condensedtherein and from 35% to 65% by weight of total resin solids present of athermoplastic resin from the group consisting of vinyl acetatehomopolymers, vinyl acetate octylacrylate copolymers, vinyl acetatevinyl chloride, copolymers, vinylidene chloride vinyl chloridecopolymers, vinylidene chloride acrylonitrile copolymers,acrylonitrilebutadiene copolymers, styrene-butadiene copolymers,interpolymers of 2-ethylhexyl acrylate, styrene, acrylonitrile andmethacrylic acid, the ratio of said photo-conductor to said resins insaid suspension, on a dry basis, being within the range of from 1 to 1to 8 to 1 parts by weight, and thereafter heating the coated substrateto efiect curing of the melamine-formaldehyde resin and produce aphoto-conductive insulating layer having the photo-conductor particlesimbedded therein, said layer being continuous and firmly bonded to thesubstrate.

3. The method as set forth in claim 2, in which themelamine-formaldehyde resin is added to an emulsion of the thermoplasticresin in water and the photo-conductor is suspended in the resultantmixture.

4. An electrophotographic recording member comprising a substrate havingfirmly bonded thereon a continuous photo-conductive insulating layercomprising electrophotographic photo-conductor particles from the groupconsisting of the oxides, iodides, selenides, sulfides and tellurides ofzinc, antimony, aluminum, bismuth, cadmium, mercury, molybdenum andlead, selenium, arsenic trisulfide, lead chromate and cadium arsenideimbedded in a resinous layer, the resinous material of said layerimparting the electrically-insulating properties thereto consistingessentially of melamine-formaldehyde resin having at least 2 mols offormaldehyde per mol of melamine condensed therein, and the ratio ofsaid electrophotographic photo-conductor particles to saidmelamineformaldehyde res-in present in said layer, on a dry basis, beingwithin the range of from 1 to 1 to 8 to 1 parts by weight.

5. An electrophotographic recording member comprising a substrate havingfirmly bonded thereon -a continuous photo-conductive insulating layercomprising zinc oxide photoconductor particles imbedded in a resinouslayer, the resinous material of said layer imparting theelectrically-insulating properties thereto consisting essentially ofmelamine-formaldehyde resin having at least 2 mols of formaldehyde permol of melamine condensed therein, and the ratio of saidelectrophotographic photoconductor particles to saidmelamine-formaldehyde resin present in said layer, on a dry basis, beingwithin the range of from 1 to 1 to 8 to 1 parts by weight.

6. An electrophotographic recording member comprising a substrate havingfirmly bonded thereto a continuous photo-conductive insulating layercomprising electrophotographic photo-conductor particles from the groupconsisting of the oxides, iodides, selenides, sulfides and tellurides ofiinc, antimony,-aluniinum, bismuth, cadmium,-

mercury, molybdenum and lead, selenium; arsenic tri-"sulfide',.leadchromateand cadmium arsenide imbedded in a resinous layerdesserts-g er from 45% to 65% by weight of total resin solids present,on a dry basis, of melamine-formaldehyde resin having at least 2 idolsof formaldehyde per mol of melamine condensed therein and from 35% to55% by weight of total resin solids present of at least one resin fromthe group consisting of vinyl acetate homopolymers, vinylacetateo'ctylacrylat'e copolymers, vinyl acetate, vinyl chloride copolymers,vinylidene chloride vinyl chloride copolymers',*vinylidene chlorideacry-lonitrile' copolym'ers; acryl onitrile fbutadie'rie" copolymers',st'yrene bntadiene copolyniers, interpolymers' of Z-ethyiheXyl acrylate,styrene, acrylonitrile and methacrylic acid, and the ratio of saidelectrophotographic' photo-conductor to said resinspresentin said layer,on a dry basis, being Within the range of from 1 to 1 to 8 to 1 parts byweight. i p v 7. An electrophotogr'aphic'recording member compris ing asubstrate having firmly bonded thereto a continuous photo-conductiveinsulating layer comprising zinc oxide photo-conductor particlesimbedded in a resinous layer consisting of from 45% to 65% by weight oftotal resin solids present, on a dry basis, of melamine-formaldehyderesin having at least 2 mols of formaldehyde per mol of melaminecondensed therein and from 35% to 55 by Weight of total resin solidspresent of at least one resin from the group consisting of vinyl acetatehomo polymers, vinyl acetate octylacrylate co'polymer's, vinyl acetatevinyl chloride copolymers, vinylidene chloride vinyl chloridecopolyrners, vinyliderie chloride acrylonitrile copolyrners,acrylonitrile-butadiene copolymers, sty-' rene-butadiene copolymers,interpolymers of 2-ethylhexyl acrylate, styrene, acrylonitrile andmethacrylic acid, and the ratio of said electrophotographic photoconduc'tor' to said resins present in saidlayer, on adry basis, beingwithin the range of from 1' to 1 to 8 to 1 parts by weight.

8. An electro'photogfaphie recording member comprising a paper basehaving firmly bonded thereto a condensed photo-conductive insulatinglayer comprising zinc oxide photo-conductor particles imbedded in aresinous layer consisting of a cured mixtureof melamine-formaldehyderesin having at least 2 mols of formaldehyde per mol of melaminecondensed therein and an interpolyrner of Z-ethylhexyl acrylate,styrene, acrylonitrile and methacrylic acid in the proportions of from35% to 55% of said interpolymer to 65% to 45% by weight of saidmelamine-formaldehyde resin, the ratio of zinc oxide to resin solids, ona dry basis, in said layer being within the range of from 1 to 1 to 8 to1 parts by Weight.

9. The method of producing an electrophotographic recording member whichcomprises coating a substrate with a suspension of finely dividedlectrophotographic photo-conductor from the group consisting of theoxides, iodides, selenides, sulfides and tellurides of Zinc, antimony,aluminum, bismuth, cadmium, mercury, molyb denum and lead, selenium,arsenic trisulfi'de, lead chro' mate and cadmium a-rsenide in an aqueousmedium containing a melamine-formaldehyde resin having at least 2r'nolsof formaldehyde" per mol of melamine condensed therein, the ratio ofsaid photo-conductor to said mel-' s'ulating-la'yer having thephoto-conductor particles em-- bedded therein, said layer beingcontinuous and firmly bonded to the substrate;

1-0. The method ofproducing an electrophotographic recording memberwhich comprises coating a substrate with a suspension of finely dividedzinc oxide photoconductor in an aqueous medium containing amelamineforrnaldehyde resin having at least 2 mols of formaldehyde permol of melamine condensed therein, the ratio of said zinc oxide to saidmelamine-formaldehyde resin in said suspension, on a dry basis, beingwithin the range of from 1' to 1 to 8 to 1 parts by weight, the saidmelamine formaldehyde resin being the essential resinous constituentofsaid aqueous medium imparting the electrically-insulating propertiestothe coating layer formed therefrom, and thereafter heating the coatedsubstrate to efiect curing of the melamine-formaldehyde resin andproduce a photo-conductive insulating layer having the zinc oxideparticles embedded therein, said layer being continuous and firmlybonded to. the substrate.

itte'r en'cesciteu in the file of this patent UNITED STATES PATENTS2,197,442 Widmer Apr. 6, 1940 2,322,888 Schwartz et al June 29, 19432,399,489 Landes Apr. 30, 1946 2,557,266. Dittmar et a1 June 19, 19512,719,831 Craemer et a1. Oct. 4, 1955 2,719,832 Craemer et al. ....aOct. 4, 1955 2,767,153 Sutton -a Oct. 16, 1956 2,875,054 Griggs et al.Feb. 27,1959

FOREIGN PATENTS 201,301 Australia Mar. 19, 1956 1,136,146 France May 9,

OTHER REFERENCES Metcalfe et al.: Journal of the Oil & Colour ChemistsAssn.,- vol. 39, #11, pages 845-856. I

Wainer: Photographic Engineering, vol. 3, #1, pages Barron: ModernPlastics, Chapmen & Hall (1949), pages 240-253.

1. THE METHOD OF PRODUCING AN ELECTROPHOTOGRAPHIC RECORDING MEMBER WHICHCOMPRISES COATING A SUBSTRATE WITH A SUSPENSION OF FINELY DIVIDEDELECTROPHOTOGRAPHIC PHOTOCONDUCTOR FROM THE GROUP CONSISTING OF THEOXIDES, IODEDES, SELENIDES, SULFIDES AND TELLURIDES OF ZINC, ANTIMONY,ALUMINUM, BISMUTH, CADMIUM, MERCURY, MOLYBDENUM AND LEAD, SELENIUM,ARSENIC TRISULFIDE, LEAD CHROMATE AND CADMIUM ARSENIDE IN AN AQUEOUSMEDIUM CONTAINING FROM 45% TO 65% BY WEIGHT OF TOTAL RESIN SOLIDSPRESENT OF A MELAMINE-FORMALDEHYDE RESIN HAVING AT LEAST 2 MOLS OFFORMALDEHYDE PER MOL OF MELAMINE CONDENSED THEREIN AND FROM 35% TO 65%BY WEIGHT OF TOTAL RESIN SOLIDS PERSENT OF A THERMOPLASTIC RESIN FROMTHE GROUP CONSISTING OF VINYL ACETATE HOMOPOLYMERS, VINYL ACETATEOCTYLACRYLATE COPOLYMERS, VINYL ACETATE VINYL CHLORIDE COPOLYMERS,VINYLIDENE CHLORIDE VINYL CHLORIDE COPOLYMERS, VINYLIDENE CHLORIDEACRYLONITRILE COPOLYMERS, ACRYLONITRILE-BUTADIENE COPOLYMERS,STYRENE-BUTADIENE COPOLYMERS, INTERPOLYMERS OF 2-ETHYLHEXYL ACRYLATE,STYRENE, ACRYLONITRILE AND METHACRYLIC ACID, THE RATIO OF SAIDPHOTOCONDUCTOR TO SAID RESINS IN SAID SUSPENSION, ON A DRY BASIS, BEINGWITHIN THE RANGE OF FROM 1 TO 1 TO 8 TO 1 PARTS BY WEIGHT, ANDTHEREAFTER HEATING THE COATED SUBSTRATE TO EFFECT CURING OF THEMELAMINE-FORMELDEHYDE RESIN AND PRODUCE A PHOTO-CONDUCTIVE INSULATINGLAYER HAVING THE PHOTOCONDUCTOR PARTICLES IMBEDDED THEREIN, SAID LAYERBEING CONTINUOUS AND FIRMLY BONDED TO THE SUBSTRATE.